1. Field of the Invention
The present invention relates to bi-directional, high-speed serial data links, and more particularly, to an enhanced high speed data link that has dual parallel termination and disconnect detection.
2. Background Information
The Universal Serial Bus (USB) is a cable bus that supports data exchange between a host computer (USB host) and a wide range of simultaneously accessible peripherals (USB devices). The terms USB devices, USB peripherals, or simply peripheral device are synonymous and cover any device that is connected to a USB cable bus and is not a USB host or hub. The USB physical interconnect is a tiered-star topology. A hub is at the center of each star. A USB cable is used to make the connection between the USB host and a hub or a USB device, or a hub connected to another hub or USB device. The USB host contains host controllers that provide access to the USB devices in the system. FIG. 1 shows a schematic view of the USB architecture. For more detailed information, the reader is invited to review the xe2x80x9cUniversal Serial Bus Specificationxe2x80x94Version 1.1xe2x80x9d published Sep. 23, 1998. The USB cable can be thought of as a transmission line. Proper termination of a data transmission line is essential to prevent reflections on the transmission line that may interfere with data transfer, particularly at high speeds. In general, a transmission line is terminated with some sort of a load resistance. A load resistance is chosen to closely match the characteristic impedance of the transmission line in order to reduce reflections that may distort the data signals.
FIG. 2 shows the electrical schematic of a USB connection as specified in the USB 1.1 specification. On the host side of the connection, a transceiver 201 drives the two signaling lines D+ and Dxe2x88x92. The transceiver 201 drives the data through a pair of impedance matching resistors RS. The impedance matching resistors RS match the output impedance of the transceiver 201 drivers to the characteristic impedance of the twisted pair signaling lines D+ and Dxe2x88x92.
Similarly, on the peripheral side of the USB cable, a transceiver 203 is provided for receiving and transmitting data signals. On the peripheral side, a pair of impedance matching resistors RS is also provided for matching the output impedance of the transceiver 203 with the characteristic impedance of the cable. Additionally, on the peripheral side of the cable, a pull-up resistor Rpu is coupled between one of the signaling lines (in this case D+) and a voltage V (typically 3 to 3.6 volts). The pull-up resistor Rpu is used to maintain proper voltage levels for data signals and to detect the presence or absence of the peripheral. All USB peripheral devices include such resistances. When the USB peripheral is connected to a host, the peripheral applies a voltage to the signaling line that permits the host to detect the presence of the USB peripheral. Typically, the value of the pull-up resistor Rpu is 1.5 kxcexa9.
Additionally, pull-down resistors Rpd are attached to each of the signaling lines D+ and Dxe2x88x92. The pull-down resistors Rpd are typically on the order of 15 kxcexa9. The purpose of the pull-down resistors Rpd is to pull the signaling line to ground if the pull-up resistor Rpu is disconnected. Therefore, the pull-down resistors Rpd are used for disconnect detection.
As noted above, the pull up resistor Rpu is used to maintain proper voltage levels on the signal lines. The pull up resistor Rpu acts as a variable current source which injects anywhere between 200 microamps to 2.4 milliamps into the data line to which it is attached. The existing USB 1.1 specification requires the pull up resistor Rpu to raise the line from 0 to 2.0 volts within 2.5 microseconds and maintain the line between 2.7 and 3.6 volts when the bus is quiescent. However, the use of the pull up resistor Rpu requires valuable board area. Thus, it is desirable to that the pull up resistor Rpu be removed.
Furthermore, existing USB connections have the capability of detecting when a USB device is disconnected. This is typically done by using the pull up resistor Rpu. This technique is impractical for enhanced high speed USB signal rates.
The design of the USB connection shown in FIG. 2 supports existing USB signaling rates. However, even higher signaling rates have been proposed. For the higher signaling rates, improved cable terminations must be provided. Additionally, the USB connection termination system and method must be backwards compatible, i.e. support current USB 1.1 signaling rates that are of lower speed. Furthermore, the enhanced USB connection should have the capability of detecting disconnects.
A method for detecting the disconnection of a peripheral device from a serial data bus is disclosed. First, a predetermined current is injected to the serial data bus in a data packet that is sent over the serial data bus. Then, the voltage level on the serial data bus is monitored.
The present invention will be described in conjunction with the following Figures, wherein:
FIG. 1 is a schematic diagram of the tiered star topology used in the USB architecture;
FIG. 2 is a schematic diagram of a prior art USB termination system and method;
FIG. 3 is an embodiment according to the present invention, and more specifically, a schematic diagram of a USB cable and termination system;
FIG. 4 is an embodiment according to the present invention, and more specifically, a schematic diagram of the current sources used to set the voltage on the signaling line for the USB cable;
FIG. 5 is an embodiment according to the present invention, and more specifically, a voltage-current graph of the current sources of FIG. 4;
FIG. 6 is an embodiment according to the present invention, and more specifically, a schematic diagram of the signaling line for the USB cable when the USB cable is connected to a USB device; and
FIG. 7 is an embodiment according to the present invention, and more specifically, a schematic diagram of the signaling line for the USB cable when the USB cable is disconnected to a USB device.